Influenza vaccines have been manufactured for over 70 years using a process that involves infecting embryonated chicken eggs with influenza virus. The process is difficult to automate, labor-intensive, costly and creates significant risk of contamination. The entire production process requires detailed planning that begins up to 8 months prior to vaccine delivery and leaves little room for error. For instance, the 2004 worldwide influenza vaccine shortage was the result of contamination at a flu vaccine manufacturing facility. As highly pathogenic strains continue to emerge and spread, the shortcomings of egg-based manufacturing are becoming even more apparent.
Another significant drawback to current vaccine manufacturing is poor virus yield. Vaccine production includes a significant fixed cost component, reaching 90% of the total early stage manufacturing costs. Once many of the fixed costs have been recovered, however, substantial long-term variable costs still remain. It is estimated that with current egg-based production methods variable costs can run as high as 37% of the price per vaccine dose depending on the volume produced. Hence, even a two-fold increase in virus yield could have substantial impact on the cost of manufacturing and the availability of supply. Rapid increases in supply would be particularly important during a regional or worldwide pandemic.
To address the issues of contamination, production time and yield, influenza vaccine manufacturers are developing cell-based manufacturing systems, such as viral production in MDCK (Madin-Darby Canine Kidney) cells, Vero (African green monkey kidney) cells and PER-C6® (embryonic human retinal) cells. A 2007 report commissioned by the Initiative for Vaccine Research at the World Health Organization recognized these as the “three leading candidates (Vero, PER.C6 and MDCK)” of “mammalian cell lines that have been documented to support sufficient replication of influenza viruses.” See “Use of Cell Lines for the Production of Influenza Virus Particles,” Peter A. Patriarca, M.D., Biologics Consulting Group, Inc., USA, commissioned by WHO, Apr. 10, 2007.
However, MDCK cells are inherently tumorigenic, while Vero and PER-C6® cells have low virus yields and can have problematic side effects. For instance, the 2007 WHO report indicated that phase II/III trials of a whole-virion influenza vaccine produced in Vero cells was “suspended due to a higher-than-expected rate of fever and associated symptoms among trial participants.” See footnote 1 at page 5 of the WHO/Patriarca report. Thus, as of 2007, government authorities recognized that the mechanism for immortalization of Vero and MDCK cells was unknown, and that therefore a vaccine developer would have to “make every effort to detect any unknown agent that could potentially be oncogenic.” See WHO/Patriarca report at page 10. Accordingly, Europe and the United States expect rigorous testing of the viral seed for extraneous agents in accordance with both Ph. Eur. monograph 2.6.16 and 21 C.F.R. §630.35. Development of a safe, high yielding mammalian cell line, therefore, would be a significant improvement to existing influenza vaccine manufacturing practices.